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Possible Duplicate:
Charge a laptop via USB power (500mA)?

I will fly to China in the next month which will take a mammoth 18 hour flight. The plane has USB power source on it.

I know USB only has 5v and 0.5 amps output and laptop is 20v and 3.25amps. So 4 USB's would be short on the amps but supply enough voltage. Is there a transformer out there that could make this set up work?

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Jefro
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    20V x 3.25A = 65W required. 5V x 0.5A = 2.5W available. 65 / 2.5 = 26 USB outlets needed. – RedGrittyBrick May 17 '11 at 14:06
  • @RedGrittyBrick - better pack lots of 10M/20M USB extension leads then. – Linker3000 May 17 '11 at 14:23
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    argh! I'd have to ask 25 other passengers if I can use their USB ports lol – Jefro May 17 '11 at 14:55
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    New device found: Airbus 380. Do you wish to configure the device now? – Simon Richter May 17 '11 at 15:00
  • You might want to see the answers to this question as well: http://superuser.com/questions/94170/can-i-charge-a-computer-through-its-usb-port-which-brand-models – CarlF May 17 '11 at 15:13
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    @Simon Richter: http://www.heise.de/ct/schlagseite/2003/1/gross.jpg – starblue May 17 '11 at 19:57
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    Since RedGrittyBrick pointed out so quickly that power requirements make this impossible (though [you don't really need that much power](http://www.codinghorror.com/blog/2006/04/how-much-power-does-my-laptop-really-use.html)), this question is probably over-and-done, but I want to clarify: *Finding* a transformer is not a good question for Electronics. We'd welcome questions about the *design* of a DC-DC converter, but we're just as adamant (if not more adamant) about prohibiting bad shopping questions as Superuser. – Kevin Vermeer May 17 '11 at 21:51
  • When your laptop is idle the USB might have enough power to charge or break even. When you do something high power... USB will not keep your battery charging BUT it is important to note that it will still help you run your halptop longer! – Trevor Boyd Smith May 18 '11 at 02:30
  • Related: http://superuser.com/questions/158398/charge-a-laptop-via-usb-power-500ma – Jay Bazuzi May 18 '11 at 05:13
  • @RedGrittyBrick 65W is enough for heavy use + charge at the same time. Wouldn't we require less for minimal usage? The user will probably be happy if he could extend his battery life from 6-8 to 16-18 hours. – Daniel Beck Jun 12 '11 at 16:43

5 Answers5

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First, if you got a transformer, you'd need more than 4 USB ports, as the power draw will be constant at 65W. As each USB provides 2.5W, you'd need at least 26 USB ports, and that's assuming that you have perfect efficiency in your transformer (which you won't). I'm pretty sure the airline won't let you have 26 USB ports. Even if they did, it's unlikely that an airline would let you plug in what would have to be a handmade transformer. And, even if they did, it is almost certain that airport security wouldn't let you through with such a transformer in your bag.

This is pretty much guaranteed to fail. There is a reason we don't power laptops by USB. My suggestion? Get an extra battery or two, and just swap them in and out as needed.

Lukasa
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    haha... I can see the faces of security now as they open my bag to find a mass of 26 usb cables all wired into a handmade box... – Jefro May 17 '11 at 14:44
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    Or, with 7 parallel blocks of 4 supplies in series you get 20V, 3.5A. This would require 4 * 7 = 28 5V supplies, but does not need the transformer. Not that that is any more feasible... – sblair May 17 '11 at 15:20
  • Note that USB ports for power may supply more than 0.5 amps, as ipods/smarthphones may draw more than that if available. My 'droid says "connect to charger, usb port doesn't supply enough power" when I run tethering sometimes. – Macke May 17 '11 at 18:20
  • It make me sad to up-vote this... – Trufa May 18 '11 at 00:29
  • since those ports are probably "dumb" power supplies, you can most likely draw a lot more power from them. – Martin May 20 '11 at 14:48
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Firstly, the power draw will NOT be constant at 65 watts. The maximum rated output of the normal laptop adapter is 65 watts.My laptop adapter is also rated at 65 watts. When not charging the battery, my laptop typically draws between 10 and 25 watts.

If you really have to draw from the USB, I'd do it by charging an intermediary battery and cycling power to the laptop in waves supply power for at least a minute. Depending on your battery chemistry, perhaps start applying power to the laptop at 80% charge and remove power at 40%. Size your intermediary battery accordingly.

You won't be able to reasonably power your laptop fully, but you will be able to power it enough to appreciably increase your run time with the use of four USB ports. (10 watts nominal). Voltage conversion is an exercise for others.

It is likely that those adapters may be "charging" type adapters that put out a full amp at 5 volts. That 5 watts from one port would still be useful and could give you a freshly charged laptop battery if you fell asleep on a long-haul flight.

Jeff Ferland
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  • Personally, I wouldn't risk it, but you're quite right, and I like your solution. +1 for you. – Lukasa May 17 '11 at 16:50
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This is going to sound a little silly but it is completely allowed by FAA regulations.

You are allowed to bring in sealed Lead-Acid batteries.
enter image description here Attach a 12v cigarette adapter to that then hook that up to a small power inverter (you can get laptop ones for fairly cheep). You can then use that to charge your battery in flight.

Here is a DIY video of how to make one.

Scott Chamberlain
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Are you sure it's just USB because many planes now have a power connector for laptops - for example:

http://www.engadget.com/2007/07/24/laptop-power-in-flight-internet-coming-to-qantas/

Quite often the airline's Web site will cover their fleet's features so it's worth taking a look - and this site (below) also has lots of useful info:

http://www.seatguru.com/articles/in-seat_laptop_power.php

Linker3000
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There's absolutely no way you can charge a laptop via USB or a USB docking station. The only way is through the AC adapter or via expansion port/dock.

Diogo
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